It's still right without 'he' though?
Nope. The pronoun can't be left out in third person.
Very helpful. Thanks
"Puhuvatko he sujuvaa viron kieltä?" should also be correct! In Finnish is often said "viron kieli" or "suomen kieli" - often "kieli" (= language) is added.
However, "viroa" is much shorter and thus, for many if not most people, the preferred way to express this.